1. FIELD OF THE INVENTION
The present invention relates to a pair of cooling rolls for a twin-roll type cooling apparatus which is suitable for use in a process for forming a thin strip directly from a molten metal through rapid solidification.
2. DESCRIPTION OF THE RELATED ART
Known in the art is a pair of cooling rolls for the process for forming a metal strip directly from a molten metal, wherein the molten metal is poured from a nozzle onto the peripheral surface of a cooling roll rotating at a high speed, so as to rapidly cool and solidify the metal on the roll surface. This process is broadly sorted into two types: namely, the single-roll process which employs a single roll; and the twin-roll process which employs a pair of rolls. In general, the twin-roll process is considered to be more suitable for production of strips of a high-silicon steel, stainless steel or Inconel, from the viewpoint of quality and shape of the surface of the product strip. Studies therefore have been conducted to carry out the twin-roll process on an industrial scale to enable continuous mass-production of such strips.
One of the problems which hamper the industrial use of the single- and twin-roll processes is that various casting defects (referred to as "surface defects" hereinafter) tend to appear in the surface of the products rapidly solidified by such processes.
For instance, in the known processes mentioned above, regions of thermally inferior contact exist between the molten metal and the cooling roll so that solidification is retarded in these regions as compared with other regions, with the result that linear or island-like surface defects are caused in the portions of the product solidified at such regions. In the worst case, an irregular mosaic-like pattern of dents, i.e., so-called dimples, is formed on the product.
In order to eliminate occurrence of surface defects, Japanese Patent Laid-Open Nos. 63-501062 and 63-215340 propose a process of producing by the single roll process a crystalline metal billet having a thickness of 10 mm or less, with the roll having grooves formed in the peripheral surface of the roll so as to extend in parallel with the direction of the circumference of the roll.
On the other hand, Japanese Patent Laid-Open No. 62-254953 discloses a process for producing a metal strip of 1 to 20 mm thickness by using a cooling roll having convexities and concavities from 10 to 200 .mu.m deep formed in the surface thereof by shot-blasting or introduction of a lattice groove.
These proposed processes ar effective in diminishing the formation of surface defects provided that factors such as the pitch and depth of the grooves formed in the roll peripheral surface ar suitably selected in accordance with the type of metal and casting conditions However, production of a thin strip of 1 mm or less thickness by a twin-roll method still suffers from a problem: namely, generation of various internal defects, as explained below.
(1) So-called shrinkage cavities or porous regions tend to be formed in the thicknesswise mid-portion of a thin strip produced by grooved rolls. The formation of such porous regions are noticeable particularly in the strip portions opposing the roll grooves. PA1 (2) Cracks tend to be developed in the regions of the strip which have contacted the groove edges, due to solidification shrinkage of the metal in the roll grooves.